As one of position specifying systems capable of specifying positions of radio communication terminals (radio communication terminals) such as, for instance, mobile phones and portable information terminals having communication functions, a position specifying system uses GPS (Global Positioning System) satellites and base stations. In this type of position specifying system, when a mobile phone is located at a place such as an outdoor place where GPS satellites may be satisfactorily retrieved, a present position of this mobile phone may be specified based upon GPS satellite signals transmitted from the GPS satellites. To the contrary, when the mobile phone is located at a place such as an indoor place where GPS satellites cannot be satisfactorily retrieved, a present position of this portable terminal may be specified based upon signal delay amounts of a base station signal transmitted from nearby base stations.
On the other hand, when a mobile phone may solely specify a present position thereof, almanac information of GPS satellites and almanac information of base stations must be stored in this mobile phone, and/or complex positioning calculations must be carried out in this mobile phone. Due to the above requirements, the mobile phone becomes considerably expensive and the structure of the mobile phone becomes complex. Under such a reason, another position specifying system has been proposed, in which a mobile phone does not solely specify a present position. That is, in this system, while the mobile phone transmits to a position information server both GPS satellite information which has been acquired by retrieving GPS satellites and also nearby base station information which has been acquired by retrieving nearby base station information, this position information server may specify a present position of the mobile phone based upon both the GPS satellite information and the nearby base station information, which are received form the mobile phone.
In such a system that the position information server specifies the present position of the mobile phone, this mobile phone determines, for instance, a nearby base station in which a signal delay amount of a base station signal is a minimum delay amount, as a reference base station among several nearby base stations which are located around a communication base station communicated with this portable terminal. Also, this mobile phone transmits the nearby base station information related to these nearby base stations to the position information server.
When the position information server receives the nearby base station information from the mobile phone, the position information server extracts both latitude and longitude of each of the nearby base stations based upon the received nearby base station information with reference to the almanac information of the base stations which has been previously held. Also, this position information server calculates quasi-distances between the own position information server and the respective nearby base stations based upon a relative relationship among the signal delay amounts of the base station signals transmitted from the respective nearby base stations, while the signal delay amount of the base station signal transmitted from the reference base station is used as a reference. Thus, this position information server predicts an approximate position of the mobile phone.
Then, the position information server sets both a code phase indicative of retrieval timing and a search window representative of retrieval time based upon the predicted approximate position of this mobile phone as a parameter which is required to retrieve for the GPS satellites by this mobile phone and also this position information server sets assist information. Then, the position information server transmits this set assist information to the mobile phone.
When the mobile phone receives the assist information from the position information server, this mobile phone extracts both the code phase and the search window from the received assist information, and then retrieves the GPS satellites so as to acquire the GPS satellite information based upon both the retrieval timing indicated by this extracted code phase and the retrieval time expressed by this extracted search window. Also, the mobile phone acquires the nearby base station information, and then transmits both the GPS satellite information and the nearby base station information, which has been acquired, to the position information server.
Then, when the position information server receives both the GPS satellite information and the nearby base station information from the mobile phone, this position information server specifies a present position of the mobile phone based upon both the GPS satellite information and the nearby base station information, which are received. Thus, this position information server transmits this specified present position of the mobile phone as a positioning result to the mobile phone.
On the other hand, since the above assist information and the like are transmitted/received via the communication base station between the mobile phone and the position information server, if a distance becomes long between the position where this communication base station is installed and the position where the potable phone is actually present, a time difference between the reference timing of the communication base station and the reference timing of the mobile phone may arise as a propagation delay error. Under such a circumstance, when the position information server predicts the approximate position of the mobile phone, this position information server considers the distance between this predicted approximate position of the mobile phone and the position where the communication base station is installed. Then, the position information server sets both a code phase and a search window, which have been corrected based upon this considered distance.
In this case, FIG. 18 and FIG. 19 represent a relationship between retrieval timing indicated by a code phase and retrieval time represented by a search window, which are set by the above position information server. In this case, this position information server considers the distance between this predicted approximate position of the mobile phone and the position where the communication base station is installed, and then, predicts reference timing of the mobile phone so as to calculate a correction value. At this time, such a time difference is expressed as a correction error between the actual reference timing of the mobile phone and the reference timing of the mobile phone which is predicted by the position information server. Then, this correction error may be produced as a time difference between the retrieval timing set by the position information server and the retrieval timing at which the mobile phone actually retrieves the GPS satellites.
In this case, when a time difference between the actual reference timing of the mobile phone and the reference timing of the mobile phone predicted by the position information server is small, as shown in FIG. 18, the correction error becomes small. As a result, the retrieval timing set by the position information server is not deviated from the retrieval time during which the mobile phone actually retrieves the GPS satellites ((d) in FIG. 18). Also, the mobile phone can retrieve for the GPS satellites at the ideal retrieval timing of the GPS satellites. Then, when the mobile phone can succeed in the retrieving operation of the GPS satellites, the position information server can properly specify the present position of the mobile phone.
However, when a time difference between the actual reference timing of the mobile phone and the reference timing of the mobile phone predicted by the position information server is large, as shown in FIG. 19, the correction error becomes large. As a result, the retrieval timing set by the position information server deviates from the retrieval time during which the mobile phone actually retrieves the GPS satellites ((d) in FIG. 19), so that, the mobile phone cannot retrieve for the GPS satellites at the ideal retrieval timing of the GPS satellites. Then, when the mobile phone fails in the retrieving operation of the GPS satellites, the position information server cannot properly specify the present position of the mobile phone.